Lead/acid batteries are "cycled" as they are repeatedly discharged and recharged. In a "shallow" cycle (for example, as in starting an automobile engine), the extent of discharge is relatively small and the discharge is followed more or less immediately by recharging (for example, as the engine drives the generator); only a small proportion of the ampere/hour capacity of the battery is used.
In some applications, however, the extent or "depth" of discharge before recharge is much greater. For example, in a battery-powered forklift truck the battery may be discharged for a number of hours (during the working day) before it can be recharged (typically overnight). The major portion of the capacity of the battery may be used before recharging commences. The discharge/charge cycle is said to be a "deep cycle" if more than about 40% of the ampere/hour capacity of the battery is drawn before the battery is recharged. "Deep cycle life" is the expected longevity of a battery which is in deep cycle use, expressed for example as the number of deep discharge cycles before battery capacity drops below a defined percentage (usually 80%) of its original capacity.
During the discharge portion of the use cycle of a lead/acid battery, the sulfuric acid electrolyte partially converts the lead peroxide (PbO.sub.2) active material of the positive plates to lead sulfate (PbSO.sub.4), and also converts the spongy lead of the negative plates to lead sulfate. The electrolyte is diluted by the loss of sulfate ions to the plates and by the water formed in the reaction: EQU PbO.sub.2 +Pb+2H.sub.2 S0.sub.4 .fwdarw.2PbSO.sub.4 +2H.sub.2 O
Reversely, in the charge portion of the cycle, lead sulfate on the positive plates is reconverted to the peroxide, the lead sulfate on the negative plates is converted back to lead, and the acid is regenerated. If the battery is overcharged, some of the electrolyte liquid is electrolyzed to hydrogen and oxygen gas, which in an unsealed battery escapes to atmosphere, and water must be added to prevent the electrolyte from becoming too concentrated. In a conventional "maintenance-free" battery (which has grids of "non-antimonial" lead), only a little gas is evolved in charging because the voltage is controlled and the current drops to a small level at the end of charge. This greatly reduces electrolysis, so that little gas is vented; hence no additions of water are required to such batteries.
In a sealed battery, oxygen gas evolved in recharging is retained within the cells and is recombined to form water: EQU 2Pb+O.sub.2 .fwdarw.2PbO EQU PbO +H.sub.2 SO.sub.4 .fwdarw.PbSO.sub.4 +H.sub.2 O
Hydrogen gas is not normally evolved in a sealed battery because the active material balance is such that the positive plates reach full charge before the negative plates.
The deep cycle life of the different types of batteries varies greatly. Maintenance-requiring batteries with very good deep cycle life are available. However, it has not heretofore been possible to produce maintenance-free batteries with good deep cycle life, at least in the large capacities and shapes needed for certain commercial and industrial applications. Thus, maintenance-free batteries have been restricted to use in applications which involve only shallow discharge cycles, for example, in automobiles. They have not been satisfactory for heavy use where good deep cycle life is requisite, as in industrial forklift trucks. By way of illustration, test maintenance-free batteries of conventional type but sized for use in a golf cart, could be deeply cycled relatively few times before their capacity started to decrease severely; and after about 50 cycles their capacity became so low that they were of little further use. Such short deep cycle life is grossly inadequate for commercial use.